1. Field of the Invention
The present invention relates to a digital optical writing system, and more particularly, to an optical scanning device for use in a digital output device, such as a printer and a facsimile machine, and image forming apparatus that uses the optical scanning device.
2. Description of the Related Art
In an optical scanning device for use in an image forming apparatus, temperature generally increases at and around an optical deflector under application of heat produced by a rotating polygon mirror. This causes an optical housing to have uneven temperature distribution (temperature difference). The temperature rise and the uneven temperature distribution in the optical housing can cause an optical component, such as a mirror, to be deformed or move a position where the optical component is fixed, thereby causing beam spot displacement on a photosensitive member and degradation in image quality to occur. The larger the temperature rise, the larger the beam spot displacement and the degree of the degradation in image quality. To this end, a structure for providing air cooling by using a fan that produces airflow and a duct that is arranged around the optical housing has been disclosed. However, this conventional structure is disadvantageous in requiring additional space for the duct around the optical housing, which inhibits compact construction of an image forming apparatus. The conventional structure is also disadvantageous in being less environmentally friendly because driving the fan increases power consumption.
The conventional structure is also disadvantageous in that because cooling is provided from the outside of the optical housing that incorporates heat sources, a large motor fan is required to obtain sufficient cooling effect. In particular, when the optical housing is made of resin of which thermal conductivity is typically low, heat is conducted to the entire optical housing before the heat reaches the outer surface of the optical housing. The conducted heat causes the optical housing to be deformed, resulting in beam spot displacement on the photosensitive member and degradation image quality.
FIG. 10 and FIG. 11 are schematic cross-sectional views of examples of the conventional structure for illustration of the disadvantages. Each of the example structures disclosed in Japanese Patent Application Laid-open No. H1-19601 (FIG. 11) and Japanese Patent No. 3192271 (FIG. 10) includes a filtered air inlet (242, 131d) that is arranged above a polygon mirror (214, 127) and an air outlet (244, 121b) that is arranged below the polygon mirror in an optical housing. However, with the conventional structures, because airflow circulates over the polygon mirror as depicted in FIG. 10 or FIG. 11, an amount of intake air is insufficient, and the pressure does not build up in the optical housing sufficiently. Accordingly, air exhaustion and cooling effect resulting therefrom have been insufficient. The conventional structures have also been disadvantageous in that fine dusts flown into the optical housing through a gap between a housing cover and the optical housing and gaseous contaminants that have passed through the filter are conveyed by the circulation flow over the polygon mirror and stick to the polygon mirror, thereby causing mirror surfaces of the polygon mirror to be hazed.
FIG. 28 is a schematic top view of an optical scanning device of another conventional structure.
The conventional optical scanning device includes a polygon mirror 622 that is substantially hermetically sealed in a polygon mirror chamber. The optical scanning device includes first reflectors 626a arranged toward a fixing device and second reflectors 626b arranged away from the fixing device. The temperatures of the first reflectors 626a and the second reflectors 626b differ from each other by approximately 6 to 7 degrees Celsius while scanning is performed. This temperature deviation causes optical elements and components that support the optical elements to thermally expand nonuniformly, which results in chromatic misregistration.
FIGS. 12 and 13 are schematic diagrams for illustrating hazed portions on mirror surfaces of a polygon mirror.
The mirror surfaces are hazed and/or become dirty as follows. As depicted in FIG. 12, when the polygon mirror rotates to bring a second surface or a third surface of the polygon mirror to a side where a first surface is currently positioned, haze and dirt mainly cover a downstream portion (for example, a right-hand portion of the second surface) of the surface. The reason why the downstream portion mainly becomes hazed and dirty is assumed as follows. When the polygon mirror rotates, air is pushed radially outward at a portion that includes an edge where air passes around (for example, a left-hand portion of the first surface) (hereinafter, “air-leaving portion”) of a mirror surfaces. As a result, as depicted in FIG. 13, a negative pressure develops over a portion on a mirror surface (the right-hand portion of the second surface) immediately downstream of the air-leaving portion. Dusts and gaseous contaminants contained in air supplied from above and below the polygon mirror are sucked to the negative-pressure portion and solidified to stick onto the polygon mirror surface. This mechanism will be described in detail below.
It has experimentally been confirmed that the polygon mirror is hazed and becomes dirty such that, as depicted in FIG. 12, when the polygon mirror rotates to bring the second or third surface to the side where the first surface is currently positioned, haze and dirt mainly cover a downstream portion (for example, the right-hand portion of the second surface) of an air-leaving portion (for example, the left-hand portion of the first surface).
More specifically, an airflow-analyzing computer simulation has been performed on airflows over the polygon mirror to track the cause of the haze and dirt on the polygon mirror surfaces. The computer simulation has revealed that when the polygon mirror rotates, air is pushed radially outward at the air-leaving portion (for example, the left-hand portion of the first surface). As a result, a negative pressure develops over the portion (the right-hand portion of the second surface) immediately downstream of the air-leaving portion, and air is supplied from above and below the polygon mirror to the negative-pressure portion.
Hence, it is estimated based on result of the simulation that the haze and dirt are mainly caused through the following process. Dusts and gaseous contaminants contained in the air above and below the polygon mirror are sucked to the negative-pressure portion, attached onto the reflector surface, and solidified (sublimation from solid to gaseous solid), thereby sticking to the mirror surface.
An example of another conventional structure is disclosed in Japanese Patent Application Laid-open No. 2006-221033. This conventional structure employs a suction air duct to prevent noise produced by a rotating polygon mirror from coming out of an optical housing. However, even when such a suction air duct is provided, wind noise produced by the rotating polygon mirror comes out of the optical housing because sufficient attenuation of the wind noise cannot be achieved only by the suction air duct.
In the structure disclosed in Japanese Patent Application Laid-open No. 2006-221033, a fan serving as a cooling unit is arranged above a rotary polygon mirror. However, because a load placed on a polygon-mirror drive motor is increased by a load required to drive the fan, an amount of electric current supplied to the drive motor increases, which increases power consumption and hence an amount of heat produced by the drive motor. In other words, when a fan is additionally provided in an optical scanning device, power consumption by the optical scanning device increases as compared with an optical scanning device without a fan, disadvantageously making the optical scanning device with the fan less environmentally friendly.
Although a field of application is different from that of optical scanning devices, another technique directed to noise reduction is disclosed in, for example, in Japanese Patent Application Laid-open No. H11-287544. This technique is directed to an apparatus that uses a suction fan and an exhaust fan having the same sound power level and acoustic frequency of noise and achieves noise reduction by causing acoustic transmission distances for the fans to differ from each other by a half-wave length of the acoustic frequency.
It is an object of the present invention to provide an optical scanning device in which air whose temperature is lower than that inside an optical housing is supplied to space around a polygon mirror, thereby increasing cooling effect and preventing mirror surfaces of the polygon mirror from being hazed and becoming dirty.
Addition of an air inlet to the optical scanning device brings about an increase in noise power level. To this end, a suction air duct that can effectively reduce noise is arranged at the air inlet. The suction air duct prevents wind noise produced by a rotating polygon mirror and noise produced by a driving motor from directly coming out of the optical scanning device.
It is another object of the present invention to prevent heat leakage to a light source and a scanning lens, thereby reducing beam spot displacement that can occur when a fixed position of an optical component is changed by heat.